The invention concerns a data transmission system for the exchange of digital data between a central processor unit and several peripheral control modules which are connected to the former by a bus system such that communication is enabled and which are provided for the purpose of activating a respective one, out of several safety devices such as airbags, belt tensioning systems, roll-over bars, or for driving actuating elements within motor vehicle control systems such as anti-lock braking systems, anti-skid acceleration control systems, or engine management systems; these peripheral control modules are each fitted with one buffer capacitor whose energy content is sufficient for operating the respective module, and the device driven by this module, within specifications over a limited period of time--in the case of an airbag, for example, for a period of 200 ms approximately.
Standard data transmission systems of this type feature a data bus by means of which data communications between the central processor unit and the peripheral modules will be effected, as well as a supply bus by means of which the supply energy will be fed to the control modules and the devices they control.
If the routing and fitting costs involved in such a "double" bus system are to be reduced, "halved" as it were, by changing over to a "single wire" bus system--ground connections excepted--, within which data communications as well as the energy supply will be effected via the same bus system, any implementation of such a data transmission system will result in the problem that the transmitter stages of the central processor unit and the peripheral control modules must be able to drift against the capacitive loads of the buffer capacitors and therefore be designed for relatively high output power values, or--in the case of a data transmission by supply current modulation--on the one hand there will be the problem that increased requirements will need to be met with regard to the low resistance of the transmission lines in order to be able to detect reliably and recognize as data signals relatively minor changes in the current, and, on the other hand, that any changes in the charge state of the buffer capacitors caused by the changes in the current will lead to voltage signals which themselves need to be distinguishable from data signals; this requires additional circuitry in the control module area, in this way largely compensating any possible advantage that may have resulted from the simplification of the bus system; in some cases total effort and cost will even increase overall and thus produce an overcompensation effect.